Field of the Invention
This invention relates generally to a system and method for detecting cracks or similar defects in a structural element and, more particularly, to a thermal imaging system that detects the propagation or growth of a crack in a structural element by collecting infrared emissions using a fiber optic cable that occur as a result of the crack generating heat as it expands.
Discussion of the Related Art
Many devices and machines have moveable parts and components that may operate in a manner that could cause undesirable wear and metal fatigue. For example, turbines, compressors and other similar machines have rotating shafts including blades, vanes and other elements disposed thereon. Operation of such machines may cause unwanted contact of the blades and other moving components with housings and other structures within the machine. This unwanted contact could be caused by many factors, such as thermal expansion, high shaft rotation speed, motor surge, etc.
One particular area of concern is the formation of cracks that may occur in turbine blades as a result of metal fatigue and stress through continuous use. Particularly, high speed rotating blades in a steam or gas turbine may cause high cycle fatigue in the blades, which could create cracks in the blade and associated blade structures, especially where the blade attaches and is mounted to a central disk. Those cracks could rapidly grow and expand undetected at any particular point in time, possibly causing catastrophic failure of the turbine. For example, certain operational parameters of a turbine, such as rotation speed, temperature, pressure, load, etc., may create variations in component fatigue possibly resulting in crack formation, where it may be desirable to identify those operating parameters so as to avoid them.
A number of technologies have been employed in the art to detect cracks and other defects in structural elements such as turbine blades. For example, it is known to heat a structural element using an external heat source that causes emissions in the infrared wavelengths from the structural element. Immediate areas around defects in the structural element generally have a different cooling rate than the non-damaged portion of the element. Also, because a crack looks like a knife edge to the planar heat pulse, and therefore no, or minimal, heat reflections occur from the crack making it difficult or impossible to see in a thermal image. A thermal or infrared imaging camera can be used to detect the infrared emissions to identify areas in the structural element having a lower temperature.
It is also known in the art to ultrasonically excite a structural element that causes edges of cracks to rub against each other and generate heat, which can be detected by a thermal imaging camera. Particularly, an acoustic thermal effect occurs when sound waves propagate through a solid body that contains a crack or other defect causing it to vibrate. Because the faces of the crack ordinarily do not vibrate in unison as the sound waves pass, dissipative phenomena, such as friction between the faces, will convert some of the vibrational energy to heat.
Another known crack detection technique employs an electromagnetic coil that induces eddy currents in the structural element that changes its pattern at a crack or other defect, which then can be detected. The coil is moved around on the structural element, and the eddy current pattern changes at a crack or other defect. The complex impedance in the coil changes as the eddy current changes, which can be observed on an oscilloscope.
It also has been proposed in the art to employ fiber Bragg grating (FBG) sensors that are formed in a fiber and reflect a narrow wavelength of an optical input beam. By positioning the FBG sensor at a location distal from the beam input, damage to the fiber between the sensor and the input will prevent the reflected wavelength from being received, possibly indicating a defect in the structure.
One technique for detecting cracks in turbine blades is to obtain blade vibration measurements and identify changes in blade vibrations, which indicates the presence of a crack. This process has proven to be successful for detecting cracks in turbine blades for millimeter size defects. However, this technique is not able to identify when the crack is actively growing.
Currently, no suitable technology is available that is able to detect the propagation or growth of a crack, and especially the growth of a crack in a turbine blade.